The invention relates to a heart pacemaker for generating a heart stimulation signal in the apparent absence of any heart stimulation signal during a given duration of time.
Generally, a demand heart pacemaker includes a detecting device for sensing and recognizing the occurrence of intracardiac voltage signals, and an interference recognition device for evaluating the intracardiac voltage signal senses to determine if it is an interference signal, and a signal generator operable to generate the heart stimulation signal a predetermined period of time after the sensing of a signal recognized as having the characteristics of heart stimulation signals.
Prior art demand heart pacemakers generally include an interference recognition device which distinguishes noise signals from heart stimulation signals by the fact that noise signals are repetitious. A typical prior art interference recognition device is described in the German Patent BD-AS 2,025,499 and includes a circuit comprising a capacitor arranged to have two discharge time constants, one by a discharge due a first resistor and the second one by discharge between a series arrangement of a diode and a second resistor. The first time constant is selected to be considerably greater than the second time constant.
The prior art interference recognition devices for heart pacemakers have presented many problems because they have difficulty in distinguishing many types of interference signals such as an interference signal lasting for a short time duration or one that is periodic or one that has an amplitude modulation within the physiological range. In addition, the prior art interference recognition devices cannot determine an interference signal at the early stages of the occurrence of the interference signal.
As a result of these deficiencies, the prior art heart pacemakers often erroneously identify an interference signal as being an acceptable intracardiac voltage signal and no heart stimulation signal is generated by the pacemaker. This loss of a heart stimulation signal creates a dangerous situation for the pacemaker patient.
Generally, interference signals of the type that can interfere with a heart pacemaker are commonplace everyday events. For example, the operation of push buttons for control devices can generate interference signals. Periodic closure of such push buttons can potentially interfere with the useful operation of prior art pacemakers. Interference signals can arise from electro-therapeutical equipment which can generate voltage signals similar in waveform to heart stimulation signals. In addition, inductively coupled interference voltages can occur in everyday life in a form of pulsed signals or amplitude modulated signals due to an electric arc welder or an electric melting furnace, particularly at the outset of operations.
The instant invention is an improved heart pacemaker due to the improved recognition of interference signals, particularly at their initial stages.